Positive electrode for lithium secondary battery and lithium secondary battery having the same

ABSTRACT

A positive electrode for a lithium secondary battery includes a positive activation material mixture that intercalates and de-intercalates lithium ions, wherein a first positive activation material having an average particle diameter D50 of from 12.5 μm to 22 μm and a second positive activation material having an average particle diameter D50 of from 1 μm to 5 μm are mixed with a weight ratio of from 95:5 to 60:40.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/410,053, filed on Apr. 25, 2006, which claims priority toand the benefit of Korean Application No. 2005-35472, filed Apr. 28,2005, in the Korean Intellectual Property Office. The entire content ofU.S. patent application Ser. No. 11/410,053 and the entire content ofKorean Application No. 2005-35472 are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a positive electrode for alithium secondary battery and a lithium secondary battery having thepositive electrode, and more particularly, to a positive electrode for alithium secondary battery capable of increasing a volume ratio of apositive electrode and maximizing a performance of the battery and alithium secondary battery having the positive electrode.

2. Description of the Related Art

Recently, in the rapid development of electronic, communication, andcomputer industries, small-sized light-weight high-performance portableelectric apparatuses such as camcorders, mobile phones, and notebook PCshave been widely used. Therefore, demands for batteries having a lightweight, a long life cycle, and high reliability have increased. Alithium secondary battery has an operating voltage of 3.7 V or more,which is three times higher than that of a nickel cadmium battery or anickel-hydrogen battery. In addition, lithium secondary batteries have ahigher energy density per unit weight than nickel cadmium batteries ornickel-hydrogen batteries. Therefore, lithium secondary batteries havebeen used as a substitute for nickel cadmium batteries ornickel-hydrogen batteries in portable electronic apparatuses.

The lithium secondary battery generates electric energy throughoxidation and reduction reactions while lithium ions are intercalatedand de-intercalated at positive and negative electrodes. The lithiumsecondary battery is constructed by using positive and negativeactivation materials capable of reversibly intercalating andde-intercalating lithium ions and charging an organic or polymerelectrolyte solution between the positive and negative electrodes.

Typically, lithium metal has been used as the negative activationmaterial for the lithium secondary battery. However, when lithium metalis used, dendrites may be formed, and the battery may explode due to ashort-circuit. Therefore, to replace the lithium metal, carbon-basedmaterials such as amorphous carbon and crystalline carbon have beendeveloped.

The positive activation material has the most important function forperformance and safety of the lithium secondary battery. A chalcogenidecompound may be used for the positive activation material. As an examplethereof, research has been carried out on a composite metal oxide suchas a composite of LiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_(1−x)Co_(x)O₂ (0<x<1),and LiMnO₂.

Among the positive activation materials, an Mn-based positive activationmaterial has advantages in that it can be easily synthesized with lowcost and generates a low level of environmental contaminants, but it hasa disadvantage of a small capacitance. A Co-based positive activationmaterial has advantages of a high electric conductivity, a high batteryvoltage, and excellent electrode characteristics, but it has adisadvantage of a high cost. A Ni-based positive activation material hasadvantages of the lowest cost and the highest discharge capacitance ofthe aforementioned positive activation materials, but it has adisadvantage in that it is not easy to synthesize.

In the recent research efforts to find better positive activationmaterials for the lithium secondary battery, much attention has beenpaid to finding a material that can be used as a substitute for LiCoO₂and that is capable of having stability at a high charge voltage of 4.2or more, a high energy density, and a long life cycle. For example,LiCoO₂, LiNiO₂ derivative compounds obtained by changing compositions ofNi, Co, and Mn in the compounds LiNi_(x)Co_(1−x)O₂ (0<x<1),LiNi_(x)Mn_(1−x)O₂ (0<x<1), and Li(Ni_(x)Co_(1−2x)Mn_(x))O₂ (0<x<1) havebeen developed (see Solid State Ionics, 57, 311 (1992), J. Power.Sources, 43-44, 595 (1993), Japanese Patent Application Publication No.H8-213015 (Sony (1996)), and U.S. Pat. No. 5,993,998 (Japan StorageBattery) (1997)). However, the positive activation materials obtained bysimply changing the composition of Ni, Co, and Mn have not yet beenfound to be a good substitute for LiCoO₂.

On the other hand, a high capacity battery using a positive electrodeformed with high composite slurry has been proposed. The positiveelectrode is constructed by dispersing a positive electrode compositeinto a solvent such as N-methyl-2-pyrolidone to form a positiveelectrode composite slurry, coating the positive electrode compositeslurry onto an aluminum foil, and drying the slurry. Generally, in orderto increases the density of the composite, a rolling process isperformed. If the density of the positive electrode composite isincreased, the capacity per unit volume is increased, so that thecapacity of the battery can be increased. However, when a rollingprocess is used, the activation material particles may be crushed ordestroyed, depending on the particle sizes or types of the positiveactivation materials, and the composite layer may peel off or becomedetached. Therefore, it is difficult to increase the density of thepositive electrode composite by a rolling process. In addition, since anelectrolyte solution cannot easily permeate into a positive electrodecomposite layer that has a high density, charge and dischargecharacteristics may be degraded. In addition, in a course of charge anddischarge cycles, the capacity of the battery may deteriorate.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a positive electrode for alithium secondary battery capable of increasing the capacity per unitvolume and maximizing the performance of the battery by increasing thecomposite density. Aspects of the present invention further provide alithium secondary battery having the positive electrode.

According to an aspect of the present invention, there is provided apositive electrode for a lithium secondary battery, the positiveelectrode including a positive activation material mixture thatintercalates and de-intercalates lithium ions, wherein, in a graphplotting particle size versus percent in a positive activation materialmixture having particles of an average particle diameter D₅₀ of from12.5 μm to 22 μm and particles having an average particle diameter D₅₀of from 1 μm to 5 μm measured by a laser diffraction particle sizeanalyzer, a peak value for a second positive activation material havingan average particle diameter D₅₀ of from 1 μm to 5 μm to a ratio of apeak value for a first positive activation material having an averageparticle diameter D₅₀ of from 12.5 μm to 22 μm is in a range of from0.07:1 to 0.20:1.

According to another aspect of the present invention, there is provideda lithium secondary battery comprising a positive electrode as describedin the preceding paragraph, a negative electrode including a negativeactivation material that intercalates and de-intercalates lithium ions;and a non-aqueous electrolyte solution.

According to another aspect of the present invention, there is provideda positive electrode for a lithium secondary battery including apositive activation material mixture that intercalates andde-intercalates lithium ions; wherein the positive activation materialmixture comprises a first positive activation material having an averageparticle diameter D₅₀ of from 12.5 μm to 22 μm and a second positiveactivation material having an average particle diameter D₅₀ of from 1 μmto 5 μm, wherein the first activation material and the second activationmaterial are mixed with a weight ratio of from 95:5 to 60:40.

According to another aspect of the present invention, there is provideda lithium secondary battery comprising a positive electrode as describedin the preceding paragraph, a negative electrode including a negativeactivation material that intercalates and de-intercalates lithium ions;and a non-aqueous electrolyte solution.

According to another aspect of the present invention, there is providedan activation material mixture that intercalates and de-intercalateslithium ions, comprising a first activation material having an averageparticle diameter D₅₀ of from 12.5 μm to 22 μm and a second activationmaterial having an average particle diameter D₅₀ of from 1 μm to 5 μm,wherein a weight ratio of the first activation material to the secondactivation material in the activation material mixture is from 95:5 to60:40

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a graph plotting particle size versus percent of a positiveactivation material mixture. The positive activation material mixture ofFIG. 1 contains a first positive activation material having an averageparticle diameter D50 of 15.1 μm and a second positive activationmaterial having an average particle diameter D50 of 3.5 μm, wherein theratio of the first positive activation material to the second positiveactivation material is 85:15 by weight. The graph of FIG. 1 furthershows that in the plot of particle sizes, there are two peaks,corresponding to the first positive activation material and the secondpositive activation material, and a minimum peak value between the twopeaks. As shown in FIG. 1, a peak value Hb corresponds to a differencebetween the highest point corresponding to the first positive activationmaterial and the minimum peak value, and the peak value Hs correspondsto a difference between the highest point corresponding to the secondpositive activation material and the minimum peak value. From the peakvalues Hb and Hs, a ratio Hs/Hb can be calculated, and a positiveactivation material mixture can be defined in terms of its ratio Hs/Hb.according to an aspect of the present invention;

FIG. 2 is a schematic view showing a construction of a lithium secondarybattery according to an aspect of the present invention; and

FIG. 3 is a graph showing densities of positive activation materialpellets as a function of the relative amount of the second activationmaterial. The pellets were produced according to Examples 2 and 7 to 9and comparative examples 7 to 9.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

An embodiment of a lithium secondary battery having a positive electrodeaccording to the present invention is shown in FIG. 2. Referring to FIG.2, the lithium secondary battery according to the embodiment isconstructed by inserting an electrode assembly 12 including a positiveelectrode 13, a negative electrode 15, and a separator 14 together withan electrolyte solution in a can 10 and sealing an upper portion of thecan 10 with a cap assembly 20. The cap assembly 20 includes a cap plate40, an insulating plate 50, a terminal plate 60, and an electrodeterminal 30. The cap assembly 20 engages with an insulating case 70 toseal the can 10.

The electrode terminal 30 is inserted into a terminal through hole whichis formed at a center of the cap plate 40. When the electrode terminal30 is inserted into the terminal through hole, a tube-type gasket 46surrounding an outer surface of the electrode terminal 30 is alsoinserted in order to insulate the electrode terminal 30 from the capplate 40. After the cap assembly 20 is assembled to an upper portion ofthe can 10, an electrolyte solution is injected through an electrolytesolution injection hole 42, and the electrolyte solution injection hole42 is closed with a cork 43. The electrode terminal 30 is connected to anegative tab 17 of the negative electrode 15 or a positive tab 17 of thepositive electrode 16, thereby serving as a negative terminal or apositive terminal.

In FIG. 2, a square shaped battery is shown. However, the presentinvention is not limited to this type, but may be a cylindrical type, acoin type, a pouch type, or other types.

Aspects of the present invention provide a positive electrode having anincreased volume density capable of increasing the capacity of a lithiumsecondary battery. In order to increase the volume density thereof,there is a need for selecting a particle size distribution that iscapable of maximizing a packing ratio between positive activationmaterial particles. However, there is a trade-off relation between themaximization of the packing ratio and the performance of the lithiumsecondary battery, so that an optimization thereof is needed.

According to an aspect of the present invention, it is possible toincrease the packing ratio between the positive activation materialparticles by mixing two types of positive activation materials havingdifferent particle size distributions in a predetermined ratio. Theincrease in the packing ratio can be tested by measuring the density ofa pellet that is formed by pressing a powder mixture of the positiveactivation materials. By calculating the density of a pellet of thepositive activation material, the density of a composite slurry of apositive electrode plate can be estimated.

According to an aspect of the present invention, a first positiveactivation material having an average particle diameter D₅₀ of from 13μm to 17 μm and a second positive activation material having an averageparticle diameter D₅₀ of from 2 μm to 4 μm may be mixed. The weightratio of the first positive activation material to the second positiveactivation material may be in a range of from 95:5 to 60:40, or, forexample, from 90:10 to 70:30.

By mixing two types of the positive activation materials havingdifferent average particle diameters D₅₀ in a predetermined ratio, it ispossible to increase the volume density of the positive electrode plateand to obtain a long life cycle of the battery. The average particlediameters of the positive activation materials and peak values thereofare measured by a laser diffraction particle size analyzer. The peakvalues measured by the analyzer are not absolute values but relativevalues. As shown in FIG. 1, differences between a minimum peak value(between two peak values for the two types of the positive activationmaterials) and the peak values for two positive activation materials aredefined as peak values Hs and Hb, respectively. The laser diffractionparticle size analyzer used to calculate the values of FIG. 1 was aMicrotrac HRA-100 Version 10.1.2-016SE, which is a wet-type instrumentusing laser diffraction. As the measurement conditions, water was used adispersion medium; particle transmittance was measured in a reflectingmode; and run time was set to 30 seconds. In addition, in themeasurement, there is no limitation on the shape of particles and therefractive index of the particles. In addition, a refractive index ofthe medium was set to 1.33.

The first positive activation material and the second positiveactivation material may be made of a lithium composite oxide capable ofintercalating and de-intercalating lithium ions. The first positiveactivation material and the second positive activation material may havethe same chemical composition and differ from each other only in theiraverage particle diameters D₅₀ or may differ from each other in chemicalcomposition as well as average particle diameters D₅₀. For example, thefirst positive activation material and the second positive activationmaterial may each be independently selected as from the group consistingof compounds of the following Chemical Formula 1 and 2.Li_(x)Co_(1−y)M_(y)A₂  [Chemical Formula 1]Li_(x)Co_(1−y)M_(y)O_(2−z)X_(z)  [Chemical Formula 2]

In Chemical Formulas 1 and 2, 0.9≦x≦1.1, 0≦y≦0.5, 0≦z≦0.5, M is anelement selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si,Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and rare earthelements, A is an element selected from the group consisting of O, F, S,and P, and X is F, S, or P.

The lithium composite oxide may be formed by mixing a lithium compound,a cobalt compound, and a compound including at least one elementselected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn,V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and a rare earth element with asmall amount of a fluoric salt, a sulfuric salt, or a phosphoric saltand sintering a mixture thereof. The mixing process may be a dry-typeprocess, a wet-type process, or any other types. A temperature ofsintering may be in a range of from 200° C. to 1000° C. The sinteringprocess may be performed in an oxygen ambience or an inert gas ambience,but the process is not limited thereto. The sintering process may berepeated for as many times as needed. After the sintering process, asuitable cooling process may be performed, and as needed, a crushingprocess may be performed. As a result, the lithium composite oxide isobtained.

As the lithium compound, lithium hydroxide, lithium carbonate, lithiumnitrate, or lithium acetate may be used. As the cobalt compound, cobaltoxide, cobalt nitrate, cobalt acetate, cobalt hydroxide, cobaltcarbonate, a nickel cobalt salt, or a nickel cobalt manganese salt maybe used. As the compound including at least one element selected fromthe group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B,As, Zr, Mn, Cr, Fe, Sr, V, and a rare earth element, a oxide, ahydroxide, a carbonate, a nitrate, or an organic acid salt thereof maybe used. As the fluoric salt, manganese fluoride (MnF₃), lithiumfluoride (LiF), nickel fluoride (NiF₂), calcium fluoride (CaF₂), or zincfluoride (ZnF₂) may be used. As the sulfuric salt, manganese sulfide(MnS) or zinc sulfide (ZnS) may be used. As the phosphoric salt, H₃PO₄may be used.

The positive electrode may further include conductive materials forimproving electric conductivity. The conductive material may include atleast one material selected from the group consisting of a graphitebased conductor, a carbon black based conductor, and a metal or metalcompound based conductor. The graphite based conductor may includeartificial graphite, natural graphite, and the like. The carbon blackbased conductor may include acetylene black, ketjen black, denka black,thermal black, channel black, and the like. The metal or metal compoundbased conductor may include tin, tin oxide, tin phosphoric acid (SnPO₄),titanium oxide, potassium titanic acid, perovskite such as LaSrCoO₃, andLaSrMnO₃, and the like. However, the present invention is not limited tothe aforementioned conductors.

The composition of the positive activation material may be in a range offrom 0.1 to 10 wt % of the electrode activation material. If thecomposition of the conductor is less than 0.1 wt %, the electrochemicalproperty may be degraded. If the composition of the conductor is largerthan 10 wt %, the energy density per unit weight is reduced.

According to aspects of the present invention, the positive electrode isconstructed by mixing a first positive activation material, having anaverage particle diameter D₅₀ of from 12.5 μm to 22 μm, and a secondpositive activation material, having an average particle diameter D₅₀ offrom 1 μm to 5 μm, at a weight ratio of from 95:5 to 60:40 to form apositive activation material mixture, dispersing a positive electrodecomposite including the positive activation material mixture, aconductive material, and a binder into a solvent to form a positiveelectrode composite slurry, coating the positive electrode compositeslurry on a positive electrode charge collector, drying the slurry, androlling the slurry with a roller press machine. In the positiveelectrode for a lithium secondary battery according to an aspect of thepresent invention, the positive electrode composite has an averagedensity of 3.75 g/cm³. The density of the positive electrode compositeaccording to an aspect of the present invention is higher than a density(3.65 g/cm³) of a positive electrode composite of a positive electrodeconstructed with a conventional positive activation material.

The binder for activation materials has the functions of softeningactivation materials, consolidating inter-bonding of activationmaterials with a charge collector, buffering the swelling or shrinkageof the activation materials, and the like. For example, the binder mayinclude polyvinylidene fluoride, copolymer of polyhexafluoropropyleneand polyvinyledene fluoride (PVdF/HFP), poly (vinylacetate), polyvinylalcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylatedpolyethylene oxide, polyvinyl ether, poly (methyl methacrylate), poly(ethyl acrylate), polytetrafluoro ethylene, polyvinyl chloride,polyacrylonitrile, polyvinyl pyridine, styrene-butadiene rubber,acrylonitrile-butadiene rubber, and the like. The composition of thebinder may be in a range of from 0.1 to 30 wt %, more preferably 1 to 10wt % of the electrode activation material. If the composition of thebinder is too small, the binding force between the activation materialand the charge collector is not sufficient. If the composition of thebinder is too large, the binding force is sufficient, but the amount ofthe electrode activation material is accordingly reduced. This isdisadvantageous to increase the battery capacity.

The solvent used to disperse the positive electrode slurry may beaqueous or non-aqueous. For example, N-methyl-2-pyrolidone (NMP),di-methyl formamide, di-methyl acetamide, N,N-di-methylaminopropylamine, ethylene oxide, tetrahydrofuran, or the like, may be usedas the non-aqueous solvent.

The positive electrode charge collector may be made of a stainlesssteel, nickel, aluminum, titanium, or a combination or alloy thereof.Alternatively, the positive electrode charge collector may beconstructed as an aluminum or stainless steel structure with a surfacecoated with carbon, nickel, titanium, or silver. As non-limitingexamples, aluminum or an aluminum alloy may be used for the positiveelectrode. The positive electrode charge collector may be in the shapeof a foil, a film, a sheet, a punched structure, a porous structure, ora foamed structure.

The negative electrode of a lithium secondary battery includes anegative electrode activation material into/from which lithium ions canbe intercalated or de-intercalated. The negative electrode activationmaterial includes carbon materials such as crystalline carbon, amorphouscarbon, carbon composites, and carbon fiber, lithium metal, lithiumalloy, or the like. For example, the amorphous carbon may include hardcarbon, cokes, meso-carbon micro-beads (MCMB) plasticized in atemperature of 1500° C. or less, meso-phase pitch-based carbon fibers(MPCF), or the like. The crystalline structure carbon may include agraphite-based material, such as, for example, natural graphite,graphite-based cokes, graphite-based MCMB, graphite-based MPCF, or thelike. The negative electrode activation material is preferably formed ofcrystalline structure carbon. More preferably, with respect to thecarbon material, an inter-planar distance d002 thereof is in a range offrom 3.35 Å to 3.38 Å, and a crystallite size Lc thereof as measured byX-ray diffraction is 20 nm or more. The lithium alloy may include analloy with aluminum (Al), zinc (Zn), bismuth (Bi), cadmium (Cd),antimony (Sb), silicon (Si), lead (Pb), tin, gallium (Ga), or indium(In).

The negative electrode is constructed by forming a negative electrodecomposite including a negative electrode activation material and abinder, dispersing the negative electrode composite into a solvent toform a negative electrode composite slurry, coating the negativeelectrode composite slurry on a negative electrode charge collector,drying the slurry, and rolling the slurry with a roller press machine.The negative electrode composite may include a conductive material.

The negative electrode charge collector may be made of a stainlesssteel, nickel, copper, titanium, or a combination or alloy thereof.Alternatively, the negative electrode charge collector may beconstructed with a copper or stainless steel structure having a surfacecoated with carbon, nickel, titanium, or silver. As non-limitingexamples, copper or a copper alloy may be used for the negativeelectrode.

The non-aqueous electrolyte of a lithium secondary battery may, inaddition to the lithium salt and the non-aqueous organic solvent,include an additive for improving charge/discharge characteristics andpreventing an overcharge. The lithium salt functions as a source forsupplying lithium ions in a lithium battery so as to enable thefundamental operation of the battery, and the non-aqueous organicsolvent functions as a medium for transferring the ions involved inelectrochemical reactions in the battery.

The lithium salt may include at least one selected from the groupconsisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiAlO₄, LiAlCl₄, LiN (C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (x and y are natural numbers), LiCl, and LiI, or amixture of two or more thereof. The concentration of the lithium saltmay be in a range of from 0.6 M to 2.0M, preferably, from 0.7 M to 1.6M.If the concentration of the lithium salt is below 0.6M, the conductivityof the electrolyte is reduced, and the performance of the electrolyte isalso degraded. If the concentration of the lithium salt is over 2.0M,the viscosity of the electrolyte increases, and mobility of the lithiumsalt is reduced.

The non-aqueous organic solvent may include at least one selected fromthe group consisting of a carbonate, ester, ether, and ketone, or amixture of two or more of these. The organic solvent should have a highdielectric constant and a low viscosity in order to increase iondissociation and facilitate conductivity of the ions. Typically, a mixedsolvent composed of two or more solvents, of which one has a highdielectric constant and a high viscosity and the other has a lowdielectric constant and a low viscosity, is used as the organic solvent.

When a carbonate based solvent is used as a non-aqueous organic solvent,a mixture, such as, for example, a mixture of a cyclic carbonate and achain carbonate may be used. For example, ethylene carbonate (EC),propylene carbonate (PC), 1,2-butylene carbonate, 2,3-bytylenecarbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylenecarbonate (VC), or the like, may be used as the cyclic carbonate. Forexample, materials having a high dielectric constant, such as ethylenecarbonate and propylene carbonate, may be used. When artificial graphiteis used as the negative electrode activation material, ethylenecarbonate may be used, for example. The chain carbonate may includedimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate(DPC), methylpropyl carbonate (MPC), ethylmethyl carbonate (EMC),ethylpropyl carbonate (EPC), or the like. For example, materials havinga low viscosity, such as dimethyl carbonate, ethylmethyl carbonate, anddiethyl carbonate may be used.

The ester may include methyl acetate, ethyl acetate, propyl acetate,methyl propionate, ethyl propionate, γ-butyrolactone (GBL),γ-valerolactone, γ-caprolactone, δ-valerolactone, ε-caprolactone, andthe like. The ether may include tetrahydrofuran,2-methyltetrahydropuran, dibutylether, and the like. The ketone mayinclude polymethylvinyl ketone, and the like.

The lithium secondary battery according to an aspect of the presentinvention may include a separator that prevents a short-circuit betweenthe positive and negative electrodes and provides a transport channel ofthe lithium ions. As the separator, a polyolefin based material such aspolypropylene and polyethylene, a multi-layered film thereof, amicro-porous film thereof, a woven textile fabric thereof, non-woventextile fabric thereof, or other well-known separator materials may beused. In addition, a film formed by coating a resin having an excellentstability on a micro-porous polyolefin film may be used.

Now, examples of the embodiments of the present invention andcomparative examples are described. The later described examples areexamples of the preferred embodiments of the present invention, but thepresent invention is not limited thereto.

The following examples compare the volume density of positive electrodeplates and pellets for mixtures of positive electrode activationmaterials having various average particle diameters D₅₀ and variousmixture ratios.

In the following examples, a first LiCoO₂ activation material, having afirst activation diameter D50, is referred to as LiCoO₂ (A) and a secondLiCoO₂ activation material, having a second activation diameter D50, isreferred to as LiCoO₂ (B).

Comparative Example 1

A positive activation powder was formed by mixing LiCoO₂ (A) having anaverage particle diameter D₅₀ of 10.2 μm and LiCoO₂ (B) having anaverage particle diameter D₅₀ of 2.3 μm with a weight ratio ofA:B=70:30. A positive electrode composite slurry was produced by addingthe positive activation powder, an acetylene black conductive material,a polyvinylidene fluoride (PVdF) binder to an N-methyl-2-pyrrolidone(NMP) solvent. At this time, a weight ratio of (positive activationmaterial):(conductive material):(binder) was 96:2:2. The slurry wascoated on an aluminum foil and subjected to drying. The dried slurry waspressed to produce a positive electrode plate for a coin battery. Inaddition, the positive activation material formed by mixing LiCoO₂ (A)and LiCoO₂ (B) with a weight ratio of A:B=70:30 was pressed to produce apellet.

Comparative Example 2

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of10.2 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5μm.

Comparative Example 3

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of10.2 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 5.0μm.

Comparative Example 4

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of12.3 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 2.3μm.

Comparative Example 5

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of12.3 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5μm.

Comparative Example 6

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of12.3 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 5.0μm.

Example 1

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of15.1 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 2.5μm.

Example 2

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of15.1 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5μm.

Example 3

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of15.1 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 5.0μm.

Example 4

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of18.8 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 2.3μm.

Example 5

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of18.8 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5μm.

Example 6

The same processes as those of Comparative Example 1 were performed, butwith mixing of LiCoO₂ (A) having an average particle diameter D₅₀ of18.8 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of 5.0μm.

Volume densities of the positive electrode plates and palletsmanufactured according to Comparative Examples 1 to 6 and Examples 1 to6 were measured, and the result of the measurements is shown in Table 1.

TABLE 1 Density of LiCoO2, LiCoO2, [Hs/ Density of Electrode A B Hb]*100Pellet Plate D50 (μm) D50 (μm) (%) (g/cm³) (g/cm³) Comparative 10.2 2.33.5% 3.68 3.65 Example 1 Comparative 3.5 1.3% 3.65 3.63 Example 2Comparative 5 1.0% 3.5 3.52 Example 3 Comparative 12.3 2.3 5.8% 3.713.65 Example 4 Comparative 3.5 2.7% 3.69 3.66 Example 5 Comparative 51.2% 3.56 3.52 Example 6 Example 1 15.1 2.3 13.2% 3.83 3.81 Example 23.5 10.3% 3.85 3.84 Example 3 5 7.6% 3.67 3.62 Example 4 18.8 2.3 18.2%3.89 3.92 Example 5 3.5 16.5% 3.93 3.91 Example 6 5 11.3% 3.62 3.68 (A:B= 70:30 weight ratio)

As shown in Table 1, when positive electrode activation materials havingdifferent average particle diameters D₅₀ are mixed with a predeterminedratio according to an aspect of the present invention, a positiveelectrode plate comprising the mixture has a high density. Inparticular, the positive electrode plates manufactured according toExamples 1, 2, 4, and 5 have a density of 3.8 g/cm³ or more, which isgreater than the density of the positive electrode plates manufacturedaccording to Comparative Examples 1 to 5. Further, it can be seen thatthe densities of the pellets manufactured with mixture powders ofpositive electrode activation materials are roughly equal to those ofactual electrode plates.

Referring to Table 1 and FIG. 1, in measurement results with respect toparticles sizes of the positive electrode activation materials measuredby a particle size analyzer using laser diffraction, differences betweena minimum peak value (between the peak values for a small-sized andlarge-sized positive electrode activation material particle groups) andthe peak values for the smaller-sized and larger-sized positiveelectrode activation material particle groups are defined as peak valuesHs and Hb, respectively. From the peak values Hs and Hb of a positiveactivation material mixture, a ratio Hs/Hb may be calculated, which maybe expressed as a ratio or as a percent (when the ratio is multiplied by100) In the Comparative Examples above, peak value ratios ((Hs/Hb)×100)have values of 5.8% or less. In contrast, in the embodiments of theExamples according to aspects of the present invention, the peak valueratios ((Hs/Hb)×100) are greater than the peak value ratios of thecomparative Examples and range from 7% to 20%.

The following examples compare the density of pellets formed frommixtures of positive electrode activation materials according to variousaverage particle diameters D₅₀ and various mixture ratios.

Example 7

A pellet was manufactured by obtaining a positive electrode activationmaterial powder by mixing LiCoO₂ (A) having an average particle diameterD₅₀ of 15.1 μm and LiCoO₂ (B) having an average particle diameter D₅₀ of3.5 μm with a weight ratio A:B=90:10 and pressing the positive electrodeactivation material powder.

Example 8

The same processes as those of Example 7 were performed, but with mixingof LiCoO₂ (A) having an average particle diameter D₅₀ of 15.1 μm andLiCoO₂ (B) having an average particle diameter D₅₀ of 3.5 μm with aweight ratio A:B=80:20.

Example 9

The same processes as those of Example 7 were performed, but with mixingof LiCoO₂ (A) having an average particle diameter D₅₀ of 15.1 μm andLiCoO₂ (B) having an average particle diameter D₅₀ of 3.5 μm with aweight ratio A:B=60:40.

Comparative Example 7

The same processes as those of Example 7 were performed, but with usingonly LiCoO₂ (A) having an average particle diameter D₅₀ of 15.1 μm.

Comparative Example 8

The same processes as those of Example 7 were performed, but with mixingof LiCoO₂ (A) having an average particle diameter D₅₀ of 15.1 μm andLiCoO₂ (B) having an average particle diameter D₅₀ of 3.5 μm with aweight ratio A:B=50:50.

Comparative Example 9

The same processes as those of Example 7 were performed, but with usingonly LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5 μm.

Densities of the positive electrode plates and pellets manufacturedaccording to Examples 2, 7 to 9 and Comparative Examples 7 to 9 weremeasured. As shown in FIG. 3, as the relative amount of LiCoO₂ (B),having a relatively small particle diameter, decreases, the density ofthe pellet increases. When the LiCoO₂ (B) is mixed into the LiCoO₂ (A)in an amount of from 10 wt % to 30 wt %, the density of the pellet ishigh.

The following examples compare life cycle of batteries containingpositive electrode materials formed from mixtures of positive electrodeactivation materials according to various average particle diameters D₅₀and various mixture ratios.

Example 10

A positive electrode composite slurry was manufactured by mixing LiCoO₂(A) having an average particle diameter D₅₀ of 15.1 μm and LiCoO₂ (B)having an average particle diameter D₅₀ of 2.3 μm with a weight ratioA:B=90:10 to form a positive electrode activation material anddispersing the positive electrode activation material powder, anacetylene black conductive material, and a PVdF binder into an NMPsolvent. A weight ratio (positive electrode activationmaterial):(conductive material):(binder) was set to 96:2:2. The positiveelectrode composite slurry was coated onto an aluminum foil, and theresulting product was subject to drying and rolling processes, so that apositive electrode for a coin type battery was formed.

By using the manufactured positive electrode plate and a lithium metalas an opposite electrode thereof, a coin type half battery wasmanufactured in a glove box. In the examples, a mixture solution ofethylene carbonate (EC) and di-methyl carbonate (DMC) (volume ratio of1:1) with 1M LiPF₆ was used as the electrolyte solution.

Example 11

The same processes as those of Example 10 were performed, but withmixing of LiCoO₂ (A) having an average particle diameter D₅₀ of 15.1 μmand LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5 μm.

Example 12

The same processes as those of Example 10 were performed, but withmixing of LiCoO₂ (A) having an average particle diameter D₅₀ of 15 μmand LiCoO₂ (B) having an average particle diameter D₅₀ of 5.0 μm.

Example 13

The same processes as those of Example 10 were performed, but withmixing of LiCoO₂ (A) having an average particle diameter D₅₀ of 18.8 μmand LiCoO₂ (B) having an average particle diameter D₅₀ of 2.3 μm.

Example 14

The same processes as those of Example 10 were performed, but withmixing of LiCoO₂ (A) having an average particle diameter D₅₀ of 18.8 μmand LiCoO₂ (B) having an average particle diameter D₅₀ of 3.5 μm.

Example 15

The same processes as those of Example 10 were performed, but withmixing of LiCoO₂ (A) having an average particle diameter D₅₀ of 18.8 μmand LiCoO₂ (B) having an average particle diameter D₅₀ of 5.0 μm.

Comparative Example 10

The same processes as those of Example 10 were performed, but with usingonly LiCoO₂ (A) having an average particle diameter D₅₀ of 11.1 μm.

Comparative Example 11

The same processes as those of Example 10 were performed, but with usingonly LiCoO₂ (A) having an average particle diameter D₅₀ of 15.1 μm.

The coin type batteries according to Examples 10 to 15 and ComparativeExamples 10 to 11 were charged with a charge voltage of 4.2 V with 1 Cat a temperature of 25° C. under a constant-current constant-voltage(CC-CV) condition. After that, under a constant current (CC) condition,the batteries were discharged with 1 C down to a voltage of 3V. Byrepeating charging and discharging 50 times, life cycles (capacitymaintenance ratios) of the batteries were measured. The result of themeasurements is shown in Table 2.

TABLE 2 LiCoO2, A LiCoO2, B Capacity maintenance ratio D50 (μm) D50 (μm)after 50 cycles (%) Example 10 15.1 2.3 80% Example 11 3.5 85% Example12 5.0 87% Example 13 18.8 2.3 75% Example 14 3.5 68% Example 15 5.0 70%Comparative 11.1 — 88% Example 10 Comparative 15.1 — 79% Example 11(Examples 10 to 15, A:B = 85:15 weight ratio)

As shown in Table 2, the life cycles of the batteries manufacturedaccording to the examples according to aspects of the present invention,particularly, Examples 10 to 12, are substantially equal to those ofconventional batteries (Comparative Examples 10 and 11).

According to an aspect of the present invention, the composite densityof a positive electrode activation material for a lithium secondarybattery may be increased, so that it is possible to increase a capacityper unit volume of a positive electrode and obtain an excellent lifecycle thereof.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

What is claimed is:
 1. A positive electrode for a lithium secondarybattery, the positive electrode including a positive activation materialmixture that intercalates and de-intercalates lithium ions, wherein thepositive activation material mixture comprises a first positiveactivation material having an average particle diameter D₅₀ of from 12.5μm to 22 μm and a second positive activation material having an averageparticle diameter D₅₀ of from 1 μm to 5 μm, and wherein in a particlesize distribution graph that plots a percentage of particles of thepositive activation material mixture having a respective particle sizebased on a total volume of the particles as a y-axis and the respectiveparticle size as an x-axis, as measured by a laser diffraction particlesize analyzer, a minimum peak value is between a first highest peakvalue corresponding to the first positive activation material and asecond highest peak value corresponding to the second positiveactivation material, a peak value Hb corresponds to a difference betweenthe first highest peak value and the minimum peak value, and a peakvalue Hs corresponds to a difference between the second highest peakvalue and the minimum peak value, wherein a ratio of Hs/Hb is from0.07:1 to 0.20:1.
 2. The positive electrode according to claim 1,wherein the first positive activation material has an average particlediameter D₅₀ of from 13 μm to 17 μm and the second positive activationmaterial has an average particle diameter D₅₀ of from 2 μm to 4 μm. 3.The positive electrode according to claim 1, wherein the first positiveactivation material and the second positive activation material have asame chemical composition.
 4. A lithium secondary battery comprising: apositive electrode including a positive activation material mixture thatintercalates and de-intercalates lithium ions, wherein the positiveactivation material mixture comprises a first positive activationmaterial having an average particle diameter D₅₀ of from 12.5 μm to 22μm and a second positive activation material having an average particlediameter D₅₀ of from 1 μm to 5 μm; a negative electrode including anegative activation material that intercalates and de-intercalateslithium ions; and a non-aqueous electrolyte solution, wherein in aparticle size distribution graph that plots a percentage of particles ofthe positive activation material mixture having a respective particlesize based on a total volume of the particles as a y-axis and therespective particle size as an x-axis, as measured by a laserdiffraction particle size analyzer, a minimum peak value is between afirst highest peak value corresponding to the first positive activationmaterial and a second highest peak value corresponding to the secondpositive activation material, a peak value Hb corresponds to adifference between the first highest peak value and the minimum peakvalue, and a peak value Hs corresponds to a difference between thesecond highest peak value and the minimum peak value, wherein a ratio ofHs/Hb is from 0.07:1 to 0.20:1.
 5. The lithium secondary batteryaccording to claim 4, wherein the first positive activation material hasan average particle diameter D₅₀ of from 13 μm to 17 μm and the secondpositive activation material has an average particle diameter D₅₀ offrom 2 μm to 4 μm.
 6. The lithium secondary battery according to claim4, wherein the first positive activation material and the secondpositive activation material have a same chemical composition.